There is a need for single and entangled-pair photon sources for use in quantum information technology. In quantum cryptography bits can be exchanged on an open optical network along with an estimate of the probability that they were intercepted on-route. This can be used to form a cryptographic key for subsequent confidential communication over a network.
Single photon sources have been described in Michler et al in “A Quantum Dot Single Photon Turnstile Device” Science 290 p 2282-2284 (2000) and Santori et al “Triggered Single Photons from a Quantum Dot” Physical Review Letters 86 p 1502-1505 (2001) which describe single photon sources which operate by optically pumping a single quantum dot.
Electrically injected single photon emitters have been proposed by J. Kim et al in Nature, 397, p 500 (1999) based upon an etched quantum dot structure and also in a theoretical paper by Benson et al, Physical Review Letters 84 p 2513-2516 (2000).
A single photon source is also described in our co-pending application GB 2380605.
In all the prior art single photon sources described above electrons and holes are either optically or electrically injected into a quantum dot. The injected carriers then recombine to emit photons.
The average duration for this recombination process is given by the radiative decay time for the corresponding optical transition of the quantum dot. Ultimately this limits the jitter in the photon emission time from the dot and the maximum operating frequency.